An intra-erythrocytic viral infection, termed piscine erythrocytic necrosis (PEN), which results in massive red blood cell destruction has recently been discovered in the Atlantic cod (Gadus morhua), the Atlantic herring (Clupea harengus) and other marine fishes. The results of initial studies indicate the existence of a unique group of similar but distinct intraerythrocytic viruses infecting a variety of marine species. PEN may be a factor influencing fluctuations in the size of populations of these commercially valuable fish. As such, it appears to offer a unique opportunity to study virus-host relationships in the marine environment, a field of investigation that is largely unexplored. PEN virus may also serve as a useful model virus for studying the mechanism of erythrocytic infection with implications for other viruses believed to be involved in hemolytic or hematopathological conditions. The specific objectives of the proposed investigation are: (a) to determine the cytological and physiochemical effects of PEN on the Atlantic cod and other marine fishes, (b) to isolate and characterize the PEN viruses and investigate virus replication in erythrocytes, (c) to examine parasites of marine species as a possible mechanism of transmission, (d) to establish marine fish cell cultures and determine the optimum cultural conditions for such cultures, and (e) to examine a variety of marine species to establish the host ranges of the PEN viruses.